Iminoborylene complexes: evaluation of synthetic routes towards BN-allenylidenes and unexpected reactivity towards carbodiimides.

J Niemeyer,M J Kelly,S Aldridge,D Vidovic,I M Riddlestone

doi:10.1039/c5dt00131e

Abstract

The synthetic and reaction chemistries of cationic iminoborylene complexes [LnM=B=N=CR2](+), which feature a unique heterocumulene structure, have been systematically investigated. Precursors of the type CpFe(CO)2B(Cl)NCAr2 (Ar = p-Tol/Mes, 5c/d) have been generated by B-centred substitution chemistry using CpFe(CO)2BCl2 and suitable lithiated ketimines - a reaction which is found to be highly sensitive to the steric bulk at both the metal fragment and the ketimino group. Carbonyl/phosphine exchange (using PCy3 or PPh3), followed by halide abstraction allows for the generation of the cationic iminoborylenes [CpFe(PR3)(CO)(BNCAr2)](+)[BAr(X)4](-) (R = Cy, Ar = p-Tol/Mes, 12c/d; R = Ph, Ar = Mes, 13d; Ar(X) = 3,5-X2C6H3 where X = Cl, CF3) which have been characterized spectroscopically and by X-ray crystallography. The reactivity of these iminoborylene systems towards a range of nucleophiles and unsaturated substrates has been investigated. The latter includes the first examples of M=B metathesis reactivity with a carbodiimide, and results in Fe=B cleavage and formation of the isonitrile complexes [CpFe(PCy3)(CO)(CNR)](+)[BAr(Cl)4](-) (R = (i)Pr/Cy, 16/17).

Highlights

The investigation of boron-transition metal complexes has attracted widespread attention in recent years
Synthesis of iminoboryl complexes In order to put our synthetic efforts towards imino-substituted systems on a comparable basis to known complexes, we initially decided to target the [CpFe(CO)2] unit as the metal fragment, given its successful use for the generation of related cationic aminoborylenes.7a,b For the construction of precursors featuring the necessary array of consecutive Fe–B–N–C bonds, we evaluated two synthetic approaches, differing in the order of formation of the relevant bonds to the boron centre (Scheme 3)
Mirroring existing synthetic routes to [CpFe(CO)2] boryl complexes,17c we initially attempted the generation of complexes of type 5 by reaction of the anionic [CpFe(CO)2]− reagent 1 with the corresponding dichloro-(imino)boranes 2, establishing the B–N connectivity prior to the formation of the Fe–B bond (Scheme 3, Scheme 2 Target synthesis of iminoborylene complexes by halide abstraction from halo(imino)boryl complexes

Summary

Introduction

The investigation of boron-transition metal complexes has attracted widespread attention in recent years. Reaction of 5d with Na[BAr f4] [Ar f = 3,5-(CF3)2C6H3] leads to the formation of the corresponding cationic borylene [CpFe(CO2)(BNCMes2)]+, as indicated by a downfield shift in the 11B NMR signal (δB = 75 ppm, cf 47 ppm for 5d).

Results

Conclusion